This invention relates to a process for the purification of dilute aqueous urea solutions contaminated with oily substances. It is particularly related to those urea solutions obtained in a urea dewaxing process of the type where a chlorinated hydrocarbon solvent is used as a diluent for the feed and a concentrated aqueous solution of urea is employed to form a urea-wax adduct. More particularly, this process relates to the purification of the aqueous phase portion of the filtrate or centrifugate (hereinafter referred to collectively as filtrate for convenience) resulting from separating the solid adduct from the liquid mixture of oil-solvent solution and aqueous urea solution.
In urea dewaxing processes normal paraffins are separated from hydrocarbon mixtures because of the ability of normal paraffins to pass within the crystalline structure of urea forming a solid adduct. After separating the adduct from the liquid components by filtration or centrifugation, it is decomposed by heat into its components in liquid form for ultimate recovery of the normal paraffins. The several processes which utilize this separation technique may be classified by the physical state of the urea when forming the adduct, to wit: (1) crystalline urea is mixed with the oil containing the normal paraffins, (2) the oil is percolated through a fixed bed of crystalline urea, (3) a dilute urea solution is contacted with the oil, or (4) a concentrated urea solution is contacted with the oil. Processes employing all four techniques are described in Fritz, "Urea Adduct Processes for n-Paraffin Recovery", in "Proceedings of the Symposium on Normal Paraffins" at page 29, European Chemical News Normal Paraffins Supplement, Dec. 2, 1966.
The subject of this invention relates to a urea dewaxing process where a concentrated urea solution is employed for adduct formation. In this dewaxing process, the hydrocarbon mixtures are diluted with oil solvents, preferably chlorinated hydrocarbons such as dichloromethane, and brought into intimate contact with a highly concentrated aqueous solution of urea to form an adduct of urea and n-paraffins. This solid phase adduct is separated from the liquid phases by filtering or centrifuging and then is decomposed at elevated temperature, often aided by the addition of water, into urea and paraffin. The liquid phases remaining as filtrate are a mixture of hydrocarbons substantially free of normal paraffins and dissolved in the oil solvent - hereinafter called "oily phase" - and an aqueous urea solution. In a settling vessel, known as a separator, the bulk of the aqueous urea solution forms the upper phase while the oily phase is the lower one. Any residual aqueous urea solution is removed from the oily phase by washing with water. The highly dilute aqueous urea solution is combined with the wash water and recycled for use in the adduct decomposition step. Before this is done though, any traces of the oily phase still present in the highly dilute urea solution must be removed. These oily substances would otherwise contaminate the n-paraffins released during the adduct decomposition and considerably affect their quality.
As this urea dewaxing process is presently practiced, metallic filters are often employed to remove the oily phase contamination present in the dilute aqueous urea solutions. As these oily substances are very greasy, they often choke the filter pores sooner than would solid contamination. Moreover, it is practically impossible to clean the filters by back flushing, so that new filters have to be inserted continually. In that respect, this filtering problem is different from known filtering problems.
It is therefore an objective of the invention to remove the oily phase contamination from the dilute aqueous urea filtrate in such a way that the filter pores are not so quickly clogged as heretofore. Another objective of the invention is to modify the contamination so that the clogged filters can be rapidly reactivated by back flushing with water. Furthermore, the invention is aimed at solving the said problems in such a manner that no additional materials are required than those which are normally used in the prior art urea dewaxing process wherein the adduct is formed from a concentrated aqueous urea solution.